The invention is related to the containment of radioactive items, and in particular to methods and apparatus for the containment, transportation, and storage or disposal of decommissioned nuclear reactor pressure vessels.
Nuclear power is now a mature technology. In the United States more than a hundred nuclear power plants have been at work for decades, providing a substantial portion of the commercial electrical power sent to the nation""s power grid.
Like most machines, however, nuclear power plants, and in particular the nuclear reactors which form the hearts of nuclear power plants, have limited useful lifetimes. And when their useful lives have expired nuclear reactors must, like other machines, be disposed of. But nuclear machines present more complex disposal problems than most non-nuclear machines. Nuclear machines, and in particular nuclear reactors, are, in significant part, radioactive, and therefore require special handling and containment. And when such machines must be moved for storage (or, in most cases equivalently, for disposal), the problems associated with their containment are compounded.
To date few nuclear reactors have been decommissioned or otherwise processed for disposal. This is particularly true of reactors of substantial size. In fact, only four nuclear reactor pressure vessels of relatively large size have been transported for disposal. These reactors are of the pressurized water (PWR) type and include reactors from the Shippingport, Trojan, Yankee Rowe and Saxton plants. And throughout the process of dismantling and removing these four reactors it was noted that improved methods and apparatus were needed.
In two of these instances, Trojan and Saxton, the reactor pressure vessel (RPV) was transported intact containing all of the highly radioactive internals components, in operating configuration. In the handling of these PWR pressure vessels it was determined that one preferred method for transporting a decommissioned RPV would be to place the RPV with its internals within another container, the container adapted to provide both shielding and structural integrity for the RPV and its contents during transportation. To date only with the Saxton RPV was this tried.
To meet the stringent requirements for safe containment during transportation and disposal of such a potentially dangerous item as the Saxton RPV, highly specialized and innovative apparatus and methods were employed. The reactor pressure vessel was disconnected from the piping which connected it to the remainder of the power plant, and placed otherwise intact, with its internal components in operating configuration, its pressure head in place, and with its inlet and outlet nozzles untrimmed, within another container, encased in grout to hold it in place within one section of the container, and transported. The containment configuration used in the Saxton decommissioning is illustrated in FIG. 1. Containment package 200 comprises RPV 202 and canister 201. Canister 201 comprises first and second sections 236 and 237, respectively. RPV 202, intact with all internals 217 (not shown in their entirety) and with reactor pressure head 215 in place, firmly attached to the RPV by means of head-to-body attachments 232, is disposed within canister 201. External fittings 203 have been severed, but substantial portions 204 of the fittings have been left in place. External insulation 206 has been left in place. Interior 209 of RPV 202 has been filled with grout, and gap 251 between RPV body 214 and the canister has been filled with grout. Surface contaminants have been sealed by grout on RPV interior surface 210 and on RPV exterior surface 205. Exterior surface 213 of canister 201 comprises no fenders or other protection against shocks.
While the containment apparatus and the methods applied in decommissioning the Saxton RPV represented an innovative approach to the problem of transporting a large radioactive waste item, it became increasingly clear to those concerned as work progressed that there was substantial room for improvement. For example, the container, being designed for containment of the entire RPV, with its internal structures, the head, external insulation, and all external fittings in place and intact, was substantially larger and heavier than necessary. The complete package measured about 21xc2xd feet in length, 9 feet in diameter, and weighed about 120 tons. And Saxton was a relatively small and light reactor vessel package. Therefore, when larger PWR and boiling water reactor (BWR) pressure vessels, often weighing in excess of 600 tons without container packaging, are considered, it becomes necessary to consider improved methods of handling and packaging.
Other approaches for the handling and transport of radioactive waste for disposal have been suggested, but none provides a truly satisfactory solution to the particular problems associated with the containment and transportation of large radioactive machinery such as RPVs of commercial size. For example, U.S. Pat. No. 5,894,134 to Kissinger, xe2x80x9cSHIPPING CONTAINER FOR RADIOACTIVE MATERIALxe2x80x9d, and U.S. Pat. No. 5,061,858 to Mallory, xe2x80x9cCASK ASSEMBLY FOR TRANSPORTING RADIOACTIVE MATERIAL OF DIFFERENT INTENSITIESxe2x80x9d, describe tubular shipping containers for radioactive material comprising impact limiters disposed at either end of the containers; U.S. Pat. No. 5,297,182 to Cepkauskas, xe2x80x9cMETHOD OF DECOMMISSIONING A NUCLEAR REACTORxe2x80x9d, discloses a method for encapsulating portions of the reactor vessel and reactor internals into a solid reactor capsule and then converting this reactor capsule into a plurality of decommissioned segments; and U.S. Pat. No. 5,263,062, Guigon et al., xe2x80x9cPROCESS AND APPARATUS FOR DISMANTLING THE INTERNAL EQUIPMENT OF A WATER-COOLED NUCLEAR REACTORxe2x80x9d, discloses a method of decommissioning a water-cooled nuclear reactor vessel where elements of the vessel are separated and compacted underwater for extraction and further transport. But the containment vessels and methods disclosed are not suitable to the safe, efficient, and economical containment, storage or disposal, and transport of decommissioned nuclear reactor pressure vessels. Their construction is more complex, and therefore more costly, than necessary; and the resultant containment package is either larger and heavier than need be or is cut at great expense and considerable risk to personnel into a number of smaller, yet still heavy, objects which must be transported and stored separately. None of these adequately addresses questions of weight, bulk, shock absorbency, or manageability for dismantlement, containment, and transport of nuclear RPVs or similar large radioactive machinery in the decommissioning process.
Thus there is a need for suitable and efficient methods and apparatus for containing, storing, and transporting nuclear reactor pressure vessels with essential intact internals components, and other radioactive machinery items, particularly during the decommissioning, dismantlement, or disposal process.
It is an object of the invention to provide methods and apparatus for containing, transporting, storing, and disposing of nuclear reactor pressure vessels and other radioactive machinery items, particularly during the decommissioning or dismantlement processes.
It is another object of the invention to provide such methods and apparatus which result in lighter and safer packages which provide adequate containment and also meet federal and state requirements for transport, storage, and disposal.
It is another object of the invention to provide such methods and apparatus which result in smaller and more easily, safely, and economically handled packages for containment, transport, storage, and disposal.
It is another object of the invention to provide such methods and apparatus which provide improved containment and shielding for radioactive portions of items stored within the items themselves.
It is another object of the invention to provide such methods and apparatus which result in packages having improved shock absorption and dissipation characteristics during transport.
It is another object of the invention to provide light, safe, durable, and effective containers for decommissioned nuclear reactors and the like, which are economically and reliably produced and easy to employ.
These and other objects are achieved by the apparatus and methods disclosed herein. The invention provides method and apparatus for containing, transporting, and storing or disposing of radioactive machinery, including in particular decommissioned nuclear reactor pressure vessels. An improved, economically-produced container is provided which allows easier handling and packaging of machinery within plants where the machinery has been installed, and which provides improved shock absorption and attenuation characteristics, especially when packaging is complete. A reactor pressure vessel or similar item is disconnected from the remainder of the plant and prepared for packaging by being reduced to the smallest practicable dimensions for packaging, transport, and storage by removing or trimming as close to flush with the exterior of the device body as is practicable and removing any removable sections of the device and any protruding external fittings. In the case of, for example, a reactor pressure vessel (RPV), the pressure vessel head is removed and insulation and other items are removed from the outside of the pressure vessel and placed inside the vessel. A storage and containment canister, optionally cut into at least two sections to ease handling and packaging, is placed near the device.
The device is placed into a lower section of the canister and sealed so as to immobilize surface contaminants, and any detached canister sections are reattached so that the canister body is complete and substantially encloses the device. The canister is filled with a support or stabilizer substance such as a grout or low-density cellular concrete to aid in sealing surface radioactive particles in place and to help support and to help secure the device inside the canister. The canister is closed, and any removed portions of the device, as for example the pressure vessel head, are optionally secured to the outside of the canister. Preferably all penetrations through the canister exterior are sealed. The packed and sealed canister is ready to be transported to a storage location or stored on site.
In one aspect the invention provides a method for containing nuclear reactor pressure vessels or the like for transportation and storage. The method comprises removing one or more external fittings from the device in such manner that unremoved portions of the fittings are trimmed as close to flush with the external surfaces of the device as is practicable, so as to minimize the size of the device to its lowest economical limits; and disposing the device within a canister. The space between the canister is filled with a stabilizer and the canister is closed.
In practicing this aspect of the invention it is advantageous to minimize the size or overall dimensions of the device to be packaged, particularly by reducing the length of any protruding attachments or fittings, so as to minimize the size and weight of the closed or completed package. Among other advantages, this reduction in size and weight eases handling, transportation, and storage of the package when it is complete. This is accomplished in part by cutting away or otherwise removing various fittings which connect the device to the rest of a plant, such as cooling water nozzles, control lines, and the like, and trimming them as flush with the body of the device as is practicable. Many devices of the type for which use of the invention is contemplated, and in particular RPVs, comprise large or robust structures which cannot conveniently or advantageously be altered or removed, and which limit the extent to which the size of a given device may be economically reduced. For example, many devices of the type for which use of the invention is contemplated comprise large openings or access features which incorporate heavy flanges, such as the head-to-body joint flanges present on many PWR and BWR pressure vessels. In containing such devices it is generally advantageous to trim external fittings and connections, such as heavy-gage cooling water nozzles and the like, as close to flush with the outer perimeters or surfaces of these features as economical or practicable, but is not practicable to remove them all the way to the surface of the device body itselfxe2x80x94although that would be preferable, if reasonably possible. In the case of reactor pressure vessels, this can mean trimming water nozzles and the like such that they are substantially flush with the head-body flange, or preferably trimmed to within the outer contours or perimeters of such flanges. In such cases the canister required for containment of the RPV must thus generally be large enough to enclose the flange, preferably and no larger. It is generally sufficient in such cases to detach the RPV from all external connections and trim the heaviest of such connections to or within the contour of the flange perimeter, for example, and to leave remaining portions of such relatively heavy or robust structures protruding somewhat from the body of the RPV. The foregoing will serve as guidance to those of ordinary skill in the art of dismantling such machinery on the trimming of external fittings in practicing the invention. Trimming in such fashion can greatly ease the physical task or cost of trimming or removing such fittings without incurring a substantial size or weight penalty in the finished containment package.
In practicing this aspect of the invention it will be noted that disposing items such as RPVs into the types of containment vessels and canisters contemplated often results in the leaving of gaps between the outer surface of the RPV, which is usually at least somewhat irregularly shaped, and the interior surface of the containment canister. It is generally advantageous in practicing this aspect of the invention to fill such gaps with a stabilizing substance. Preferably this stabilizer is poured into any such gaps in an unset liquid state, so as to flow into and substantially, or preferably completely, fill the gaps. Such stabilizers comprise any subtances which after setting are capable of supporting a substantial portion of the load imposed by the enclosed device on the container under the types of conditions to which the container package is likely to exposed, such as during transportation conditions, or of attenuating a shock administered to the canister so that any damage to the contained device is minimized, and preferably aid in immobilizing contaminants located at or near the various surfaces of the contained device. Such stabilzers include, for example, both high- and low-density grouts or concretes. It is to be understood that while such substances will in many instances not completely fill gaps of the type which are likely to appear in such packages, they can be expected to fill such gaps substantially enough to serve the purposes described. This is what is meant by saying tht such gaps are substantially filled. In addition to providing the structural benefits described, preferred stabilizers absorb radiation, so as to aid the shielding function of the canister, in addition to providing structural support. One example of a suitable class of stabilizers comprises low density cellular concretes (LDCCs) having densities of between 25 and 90 pounds per cubic foot (pcf), and having strengths varying between 800 and 1,200 pounds per square inch. Such concretes are well known in the industry. For use between the canister interior and the RPV exterior, nominal 70 pcf concretes are preferred; for use within the RPV, to immobilize contaminants and secure various internal fittings and items stored inside the RPV, nominal 30 pcf concretes are preferred.
Optional and generally preferred additional features of this method aspect of the invention comprise disposing various removed components, such as external fittings, within the interior of the device body, as for example the RPV interior; coating interior and/or exterior surfaces of the device body with sealants to immobilize surface contaminants, and sealing the canister after it has been sealed with a metalizing spray.
A particularly advantageous means of reducing the size of the packaged item and therefore the overall size and weight of the complete containment package is to remove portions of the device to be contained from the exterior of the device, and to place them within open spaces within the device prior to closing of the containment canister. For example, portions of water nozzles, control structures, and insulation removed from the exterior of a RPV are placed inside the pressure vessel, often in space vacated by the removal of especially-highly radioactive components such as for example portions of the core barrel, the core baffle assembly, and the lower core plate, which require storage or disposal separate from the bulk of the RPV.
Sealants suitable for immobilizing surface contaminants in accordance with this aspect of the invention comprise paints, epoxies, resins, and any other substances capable of fixing radioactive substances in place on surfaces of nuclear RPVs or other contained devices. Such sealants are well known in the industry.
The use and various types of metalizing sprays suitable for sealing the canister package after the canister has been closed are well known, and their selection and employment will not trouble those having ordinary skill in the relevant arts. One example of a metalizing spray suitable for use with this aspect of the invention are the sprays developed for corrosion control and available from the International Metalizing Corporation of Cherry Hill, N.J. Such sprays include zinc, lead, nickel, monel, aluminum, copper, bronze, brass, stainless steel, hastelloy, titanium, babbit, and inconel sprays. The selection of suitable sprays will depend upon the materials used in fabrication of the canister, but appropriate combinations are well known and will not trouble designers of ordinary skill once they have been acquainted with this disclosure. The use of metalizing sprays in sealing canister penetrations ensures integrity of the containment boundary between the contents of the containment package and the environment.
An additional particularly advantageous means of reducing the size of the packaged item and therefore the overall size and weight of the complete containment package is to remove portions of the device to be contained, and in particular any relatively large portions having low radioactivity, and attaching them to the exterior of the canister. For example, it is extremely useful in packaging pressure vessels to remove the RPV head, which is typically both large and not highly radioactive, and to fix it to the exterior of the canister. Preferably any contact points or openings between the canister exterior and exposed portions of the contained device are sealed, such as for example through the use of metalizing sprays as described herein. The removal of a reactor pressure vessel head and attachment of it to the exterior of the containment package can, for example, reduce the size and weight of the package by 10-20 percent or more, without unacceptable increases in release of radiation to the environment. The selection of portions of items to be contained for attachment to the exterior of the containment package will be well within the ability of those skilled in the art, once they have been armed with this disclosure. Typically guidance will be offered by national or regional guidelines for the storage, transportation, or disposal of radioactive items. Typically levels on the order of 0.5 curies or less are appropriate, although sometimes higher levels are appropriate; depending upon local health and enironmental standards and requirements.
It may be seen that a preferred and especially useful application of the methods and apparatus of the invention comprises removing a head, at least a substantial portion of all protruding external fittings, and at least a substantial portion of external insulation from a nuclear reactor pressure vessel; removing a portion of the reactor""s internals from the interior of the pressure vessel; relocating or disposing at least some of the removed external fittings, insulation, and internals within the interior of the pressure vessel; sealing any penetrations in the pressure vessel body, such as penetrations for cooling water nozzles, control structures, or the like; disposing the pressure vessel within a container; closing and sealing the container, and attaching the RPV head to the exterior of the container. Optionally some portion of the insulation, external fittings, etc., are left out for separate disposal. Similarly, the most highly radioactive portions of the reactor internals can be removed, contained, and shipped separately in a substantially smaller and more easily handled package than the main RPV body. In practicing this aspect of the invention it is suggested that the RPV head removed and upper internals removed, as for fueling the reactor, with the reactor coolant or other liquid left in place within the RPV body so as to act as shield against radioactivity. After the upper internals have been removed the coolant or other liquid is drained to the level of the water inlet or outlet nozzles, the nozzles are plugged, and the remainder of the coolant or other liquid is drained. An additional shield, such as a steel plate, is then placed atop the remaining unremoved portions of the reactor internals, at least a portion of the removed materials are placed within the RPV cavity, and the container is closed, preferably after the addition of grouts or other sealants as described herein, both to immobilize surface contaminants and to fill gaps between the RPV body and the containment canister, to act as shielding and to provide shock absorption and attenuation. Preferably the canister is sealed, as through the use of a metalizing spray or other means such as welding, and is ready for transportation and storage.
In another aspect the invention provides apparatus for the containment, transportation, and storage of decommissioned nuclear reactor pressure vessels and the like. The apparatus comprises a canister adapted for reception of a nuclear reactor pressure vessel. The canister has a body, an interior, a closed end, and means for closing the canister, preferably in the form of an attachable lid. The interior of the canister is adapted, generally by virtue of its size and shape, to receive the radioactive item, or at least in general the most highly radioactive portions of the item. The body is generally tubular, and often elongated, so as to accommodate radioactive machinery of the type contemplated herein, and especially nuclear reactor pressure vessels, particularly of the PWR and BWR types, which are generally of elongated and substantially cylindrical shape. In saying that the body is tubular it is not intended to limit this aspect of the invention solely to canisters of substantially circular cross section. Preferred canisters are circular or elliptical in cross section, because they are fabricated of sheet or plate materials, and in particular metals such as steel, and the fabrication of circular or elliptical tubes from sheet or plate material can be particularly economical, but as will immediately occur to those skilled in the art of such devices a great many non-circular or non-elliptical cross sectional shapes will serve as well, depending upon the size and shape of the radioactive item to be stored within the canister, the method of transportation, and particular storage or disposal requirements. Square, rectangular, triangular, and other cross-sectional shapes are regarded as falling within the scope of the invention, so long as they function and may be fabricated substantially as contemplated and disclosed herein.
Preferred canisters according to the invention comprise integral sacrificial fenders located adjacent either or both ends of the canister body for attenuating shocks administered to the canister. Preferably the fenders comprise simple extensions of the canister body, formed by rolling or otherwise forming the body from sheet or plate material and adding top and bottom closures removed from the top and bottom edges of the body, so that portions of the body extend past the closures. These body extensions act as fenders by attenuating shocks, etc., administered to the canister by bending. Bending of the fenders to absorb the energy thus imparted to the canister can be either elastic or plastic in nature, or both. Fabrication of fenders in this manner is very economical, as no additional parts, forming, joining, or other processing is required, and represents a marked improvement over state of the art fenders, which comprise specially-fabricated separate parts which must be made and attached at added expense, with additional labor. Optionally fenders according to the invention are long enough to provide protection not only to the containment package as a whole, and in particular to the packaged radioactive item, but also to any additional items, such as removed reactor vessel pressure head, attached to the exterior of the canister. Canisters having fenders of the type described and adapted for reception of large commercial RPVs have been analyzed to show compliance with United States Department of Transportation (USDOT) requirements for the shipping of radioactive items. In particular, the canister designed for reception and transport of the Connecticut Yankee RPV has been analyzed for compliance with USDOT one-foot horizontal drop and one foot drop with two-feet slapdown on either end criteria, and shown to satisfy all requirements.
Canisters according to this aspect of the invention are advantageously fabricated in single-piece lengths adequate for reception of the entire radioactive item to be contained. However, it has been found that canisters of such length, and particularly those adapted for the containment of commercial PWR and BWR pressure vessels, are often unnecessarily unwieldy and difficult to handle within the confines of plants of the type in which such devices are typically installed. An advantage of canisters according to this aspect of the invention is that they are easily separable into two or more sections, as for example by conventional mechanical cutting methods, with one section being placed near the radioactive item selected for containment so that the item may be placed within the canister body with a minimum of handling, and in particular without being lifted very high. Preferred methods for separating the canister into separate sections provide for severing the sections and leaving a clean, relatively straight edge for later mating and rejoining. Once the item has been placed within the canister body in this fashion any detached portions of the canister may be repositioned and rejoined, as for example by welding. As will occur immediately to those skilled in the joining of structures in the manner described herein, reinforcing or backing plates or doublers may be used to ensure the structural strength and integrity of the rejoined canister structure.
A particular advantage of fabricating canisters according to this aspect of the invention as integral wholes, then cutting or otherwise separating and loading them and reattaching them is that the various sections, on being rejoined, are already sized for perfect and immediate fit, regardless of the cross-sectional shape of the canister, since they were originally formed as a single, integral whole. This offers substantial savings in labor and costs when compared to alternatives comprising mechanically more complex joints, with necessarily more rigorous size and shape controls or realignments to ensure proper mating on reassembly.
Canisters according to this aspect of the invention preferably serve both as primary structure for lifting, handling, and transport of the radioactive items packaged within them, and as the primary containment boundary between the radioactive items, including internal structures such as fuel rods, core parts, and the like, and the environs. This can be accomplished by using any of a wide range of known materials, including structural steel. Any materials having sufficient radioactive shielding and strength qualities will serve. The selection of suitable materials and gages will not trouble designers of ordinary skill in the art, once they have been armed with this disclosure.
An optional and particularly advantageous feature of canisters according to the invention is the adaptation of one or more portions of the exterior of the canister, and in particular the canister lid, for attachment of one or more portions of the radioactive items to be stored within them. For example, advantages offered by attachment of the pressure head of a reactor pressure vessel to the exterior of the containment canister have been discussed. Optionally, attachment of such portions to the exterior of the canister can be accomplished by using existing attachment structures on the stored item or portions thereof. For example, in the Connecticut Yankee decommissioning it has been proposed by the inventor herein that the RPV head be attached to the canister exterior through the use of existing head-to-body attachment or closure studs provided on the head flange of the pressure vessel. The canister lid is provided with holes suitable for passage of the closure studs, so that the stud ends protrude from the closed containment package, and the head is placed back upon the closure studs and bolted back in place, with the canister lid sandwiched between the head and the RPV. This not only provides secure attachment of the head to the exterior of the containment package, but additional support for the RPV itself inside the canister as well. In many instances it is suitable and advantageous for attachment of the low radioactive part to the exterior of the containment canister to the interior by use of existing attachment structures to comprise the sole attachment between the stored waste item and the canister.
Optional additional features of containers according to this aspect of the invention include one or more secondary shields, either disposed circumferentially about the inner or outer periphery of the canister or located entirely within the canister, and even within the interior of the packaged radioactive item, so as to provide internal shielding between portions of relatively high radioactivity and the environs, or between regions of relatively high and relatively low radioactivity, especially during loading or packing operations. In the latter case, the location of the shield can vary, depending upon the contents of the container package and/or the stored item.
Additional optional features of containers according to this aspect of the invention include stabilizers for disposition between the waste item and the canister, and for placement inside the waste item interior, as herein described; and attachment of trunnions and/or other rotating fixtures for turning the canister about its longitudinal axis and so ease handling during and after the packing process. An especially simple and economical means of providing for such fixtures is by attaching the lower canister atop a truncated cylinder or hoop rotating fixture of rolled or otherwise formed sheet or plate material similar to that used to manufacture the canister, the fixture diameter of a dimension suitable to permit the fixture to fit within the sacrificial fender extending from the canister body and the bottom edge of the fixture provided with wheels; the canister is left to rotate freely atop the fixture and a secure mating is assured through simple gravitational means. The fixture is removed by simply lifting the canister off and disconnecting the fixture from the canister.
In another aspect the invention provides a method of fabricating a containment vessel as disclosed herein, for use in removing a nuclear reactor pressure vessel or other radioactive waste item from a reactor plant, the method comprising the steps of fabricating a tubular body of sufficient cross-sectional dimension and length to accommodate the waste item from a structural sheet or plate material (for purposes of this disclosure the terms xe2x80x9csheet and platexe2x80x9d are synonymous); closing an end of the tubular body; dividing the tubular body into a plurality of sections, the sections of suitable length for allowing at least one section, preferably the section having the enclosed end, to be disposed in a position proximate the installation location of the waste item to facilitate placement of the waste item within the closed section; and thereafter disposing a second section of the tubular body adjacent the closed section, so that the sections may be rejoined or otherwise reattached.
It is important to note that except as inherently required by the processes themselves, the order in which the steps of the methods disclosed and claimed herein is of no significance. In many instances it is possible to change the order of the steps. Those cases in which it is or is not possible to alter the order of steps disclosed or claimed herein will be apparent to those of ordinary skill in the relevant arts as they contemplate carrying out the processes.